In order to solve the problem of being dependent on fossil energies, and to reduce the pollution associated with the implementation of chemical reactions, it proves to be of increasing interest to develop processes for producing chemical compounds of interest from renewable natural resources, which are environmentally friendly. Thus, the obtaining of synthons, in particular of phenolic synthons, derived from biomass, as platform molecules for the chemical industry, has become an important challenge for this industry.
In particular, numerous studies have been carried out for a few years in order to take advantage of the advantageous properties of natural compounds, such as plant polyphenols, which are capable of constituting substitutes for phenolic petrochemical compounds, such as bisphenol A. Among them, condensed tannins, also known as proanthocyanidins, are the most abundant in plants, in particular terrestrial plants, after lignin. These compounds are found in particular in numerous varied available natural resources such as food-processing residues, for example in fruit marcs, and unexploited biomass, in particular in the bark, leaves and needles of trees, grapevine, fruit, etc.
Condensed tannins are oligomers or polymers of polyfunctional polyaromatic monomers. These monomers belong to the class of flavan-3-ols, of general formula:

wherein Rx represents a hydrogen atom or a hydroxyl group, and Ry represents a hydrogen atom or a gallate group of formula:

Through depolymerization of condensed tannins, (+)catechin and (−)epicatechin monomers, and also derivatives substituted mainly at C4 and potentially at C2, are in particular obtained. The C2, C3 and C4 carbon atoms of these derivatives are asymmetrical, and stereoisomers other than those initially present in the structures of tannins can also be formed during the depolymerization reaction. The aromatic nature of these monomers combined with the presence of free hydroxyl functions gives them properties which make them preferred candidates for constituting starting reagents for the industrial synthesis of active products, for numerous and varied applications.
Thus, various processes for depolymerizing condensed tannins, aimed at obtaining therefrom the constituent monomers, termed terminal units, and/or derived monomers, termed extension units, have been proposed by the prior art, essentially for analytical purposes.
It has in particular been shown that condensed tannins can be depolymerized in an acidic medium by means of sulfur-bearing nucleophiles. Such a prior art can for example be illustrated by the various publications by Chen et al., 2009; U.S. Pat. No. 8 088 419; Roumeas et al., 2013; and Selga et al., 2004, which describe the depolymerization of biobased condensed tannins by thiolysis. Such an “analytical” depolymerization process makes it possible to obtain, with a high yield, flavonoid derivatives that are of use for a wide range of applications, for example of general formula:

in which R represents (CH2)2—OH, CH2—COOMe, (CH2)2—NH2 or CH2—CH(NH2)—COOH.
However, the derivatives of this type are unstable under alkaline conditions, that is to say typically at a pH above 8.5, in which conditions the sulfur-bearing group becomes nucleofugal. Thus, it has been noted by the present inventors that, at a pH equal to 9, the derivatives derived from a depolymerization carried out by means of 2-mercaptoethanol as nucleophile are more than 95% degraded after 19 h at ambient temperature. Advantage has in particular been taken of this property in order to create novel derivatives resulting from tannin depolymerization by thiolysis (Chen et al., 2009).
However, it has been noted by the present inventors that the instability of the sulfur-bearing depolymerization derivatives under alkaline conditions is prejudicial to the implementation of certain subsequent reactions for functionalizing the hydroxyl groups of the phenolic nuclei, which are necessary for certain applications.